This invention relates to a solid-state image pick-up device and a method of controlling a solid-state image pick-up device and, more particularly, to a solid-state image pick-up device having resistive gate vertical charge transfer units and a method of controlling thereof.
An inter-line type charge coupled device is a typical example of the solid state image pick-up device. The inter-line type charge coupled device comprises a photo-diode array, vertical shift registers and a horizontal shift register. The photo-diode array has a plurality of columns of photo-diodes, and the vertical shift registers are interposed between the columns of photo-diodes. A charge transfer region and transfer electrodes over the charge transfer region form the vertical shift register, and a charge transfer signal is supplied to the transfer electrodes so as to sequentially change the potential level under the transfer electrodes, and the vertical shift registers convey all the charge packets or every other charge packet from the associated photo-diode columns to the horizontal shift register.
The vertical shift register transfers the charge packets from the stage to stage, and is expected to accumulate all the charge packets supplied from the associated photo diode column. However, when the cell is shrunk, it becomes impossible to give sufficient capacitance to thereto.
One of the approaches to solve the problem is disclosed by Hendric Heyns et. al. in xe2x80x9cThe Resistive Gate CTD Area-Image Sensorxe2x80x9d, IEEE Transaction on Electron Devices, vol. ED-25, No. 2, pages 135 to 139, February 1978. According to the paper, a constant potential difference is applied between both ends of the resistive gate so as to create a gradient charge transfer channel along the resistive gate, and a charge packet is transferred through the gradient charge transfer channel. The charge transfer is carried out for each row of photo diodes, and each vertical charge transfer element is expected to transfer the charge packet from one photo diode. For this reason, it is possible to decrease the area assigned to the vertical charge transfer element. This results in enlargement of the area assigned to the photo-diode.
FIGS. 1 and 2 illustrate the prior art area image sensor having the resistive gate charge transfer devices or elements, and FIGS. 3 and 4 illustrates the vertical charge transfer elements and photo diodes. A photo-shield plate is removed from the layout shown in FIGS. 1 and 3 and the structure shown in FIG. 2 for better understanding. The prior art area image sensor is fabricated on a p-type semiconductor chip 1, and photo diodes 2 and n-type charge transfer regions 3 are formed in the surface portion of the p-type semiconductor chip 1. The photo diodes 2 have a MOS (Metal-Oxide-Semiconductor) structure, and the photo diodes 2 are arranged in rows and columns. The columns of photo diodes 2 and the n-type charger transfer regions 3 are alternately arranged, and each columns of photo-diodes 2 is associated with one of the n-type charge transfer regions 3. The n-type charge transfer regions 3 are hatched in FIG. 3. Heavily doped p-type channel stoppers 4 electrically isolate the photo diodes 2 from non-associated n-type charge transfer regions 3, and provide p-n junctions for generating photo charge. The channel potential is designed to be or the order of 2 volts.
The major surface of the p-type semiconductor substrate 1 is covered with an insulating layer 5, and a resistive gate electrode 6 of highly resistive polysilicon is patterned on the insulating layer 5. The resistive gate electrode 6 has gradient potential electrode portions 6a superposed over the n-channel charge transfer regions 3 and common electrode portions 6b/6c connected between the gradient potential electrode portions 6a and constant potential sources 7a/7b. The constant potential source 7a applies high potential level through the common electrode portion 6b to the gradient potential electrode portions 6a, and the other constant potential source 7b applies low potential level through the other common electrode portion 6c to the other ends of the gradient potential electrode portions 6a. As a result, gradient potential takes place along the gradient potential electrode portions 6a. The gradient potential electrode portion 6a, the insulating layer 5 and the n-type charge transfer region 3 form in combination each vertical charge transfer element.
The resistive gate electrode 6 is covered with an insulating layer 8, and accumulation electrodes 9 are patterned over the insulating layer 8. The accumulation electrodes 9 extend in perpendicular to the gradient potential electrode portions 6a, and are respectively associated with the rows of photo diodes 2. Each of the accumulation electrodes 9 is held in contact with the insulating layer 5 over the photo diodes 2 of the associated row at intervals, and image-carrying light is incident onto the depletion regions of the photo diodes 2. The incident light generates charge packets, and the charge packets are accumulated in potential wells under the accumulation electrodes 9 held in contact with the insulating layer 5.
The accumulation electrodes 9 are connected to a vertical shift register 10, and are selectively driven to a read-out potential level. When the vertical shift register 10 changes one of the accumulation electrodes 9 to the read-out potential level, charge packets are read out from the photo diodes 2 of the associated row to the n-type charge transfer regions 3, respectively, and the gradient potential in the electrode portions 6a moves the charge packets toward a horizontal charge transfer element 11.
Transfer gate electrodes 12a/12b extend over the n-type charge transfer regions 3 in the vicinity of the horizontal charge transfer element 11, and an accumulation electrode 13 extends between the transfer gate electrodes 12a/12b. The accumulation electrode 13 is covered with the insulating layer 8, and is spaced from the gradient potential electrode portion 6a, and the transfer electrodes 12a/12b are provided on both sides of the accumulation electrode 13.
The accumulation electrodes 9 and the transfer electrodes 12a/12b are covered with a transparent insulating layer 14 (see FIG. 4), and a photo shield layer 15 of aluminum is patterned on the transparent insulating layer 14. The photo shield layer 15 has openings 15a, and the photo diodes 2 are exposed to the openings 15a. The photo shield layer 15 prevents the n-type charge transfer regions 3 from the incident light.
The n-type charge transfer regions 3 are connected to anti-blooming drain regions 16, and an anti-blooming electrode 17 sweeps excess photo charge from the n-type charge transfer regions 3 to the anti-blooming drain region 16. The horizontal charge transfer element 11 is connected to an output circuit 18, and an image signal is output from the circuit 18.
The potential difference between the common electrode portions 6c and 6b produces a gradient potential along the gradient potential electrode portion 6a, and the gradient potential makes the potential well gradually high toward the transfer gate 12a. A charge packet CP is transferred from a photo diode 2 to the n-type charge transfer region 3, and is transferred along the n-type charge transfer region 3 due to the gradient potential level. The transfer gate 12a firstly makes the potential level thereunder high, and the charge packet CP is accumulated in the potential well under the accumulation electrode 13. Thereafter, the transfer gate 12b makes the potential level thereunder high, and the accumulation electrode 13 makes the potential level thereunder lower than the potential level under the transfer gate 12b. Then, the charge packet CP flows into the horizontal charge transfer element 11. The horizontal charge transfer element 11 transfers the charge packet CP to the output circuit 18, and the output circuit 18 converts the charge packet CP to corresponding output potential.
If the gradient potential electrode portion 6a is 4 millimeters and the potential difference between both ends of the channel created under the electrode 6a is 10 volts, the vertical charge transfer element transfers all the charge packets within 20 microseconds. The time period of 20 milliseconds is shorter than the horizontal sweeping time of 63.5 microseconds defined in the NTSC standards.
FIG. 6 illustrates a charge transfer operation of the prior art area image sensor. P-STE, P-AB, P-TGA, P-Select, P-STG, P-TGB and P-H represent a potential signal applied to all the accumulation electrodes 9 for changing the potential level of the accumulating wells in the photo diodes 2, an anti-blooming signal applied to the anti-blooming electrode 17, a potential signal applied to the transfer electrode 12a, a row selecting signal selectively applied to the accumulation electrodes 9, a potential signal applied to the accumulation electrode 13, a potential signal applied to the transfer electrode 12b and a charge transfer signal applied to the gate electrodes of the horizontal charge transfer element 11, respectively.
The potential signal P-TGB is changed from the low level VL-TGB to the high level VH-TGB, and is maintained at the high level VH-TGB in time period T1. The potential signal P-TGB at VH-TGB makes the potential level thereunder high, and the charge packets read out in the previous horizontal blanking period are transferred to the horizontal charge transfer element 11. The potential signal P-TGB is recovered to the low level, and the potential well under the accumulation electrode 13 is electrically isolated from the horizontal charge transfer element 11.
Image-carrying light is fallen onto the photo diode array 2, and the photo diodes 2 generate photo carrier in proportional to the intensity of pieces of image-carrying light, and the photo carrier is accumulated therein.
The anti-blooming signal P-AB is changed from the high level VH-AB to the low level VL-AB at time T2, and the n-type charge transfer regions 3 are isolated from the anti-blooming drain regions 16. The potential signal P-TGA is also changed from the low level VL-TGA to the high level VH-TGA at time T2, and the potential barrier is removed from between the n-type charge transfer regions 3 and the potential wells under the transfer gate 12a. 
The row selecting signal is changed from the high level VH-Select to the low level VL-Select at time T2, and the photo carrier is read out from the selected row of photo diodes 2 to the n-type charge transfer regions 3 as charge packets. The row selecting signal P-Select is recovered to the high level VH-Select. The gradient potential transfers the charge packets along the n-type charge transfer regions 3, and the charge packets are accumulated in the potential wells under the accumulation electrode 13. The potential barrier under the transfer gate 12b does not allow the charge packets to flow into the horizontal charge transfer element 11.
The charge transfer signal P-H is repeatedly applied to the gate electrodes of the horizontal charge transfer element 11 during time period T3, and the previous charge packets are transferred to the output circuit 18.
While the previous charge packets are being transferred to the output circuit 18, the potential signal P-TGA is recovered to the high level VH-TGA at time T4, the anti-blooming signal P-AB is concurrently changed to the low level VL-AB, and the potential signal P-STE is also changed to the low level VL-STE at time T4. The potential barrier under the transfer gate 12a isolates the potential wells under the accumulation electrode 13 from the n-type charge transfer regions 3, and the n-type charge transfer regions 3 are connected to the anti-blooming drain regions 16. The potential wells in the photo diodes 2 become shallow, and excess photo carrier is swept into the n-type charge transfer regions 3. The gradient potential transfers the residual photo carrier along the n-type charge transfer regions 3, and the excess photo carrier is swept into the anti-blooming drain regions 16.
The potential signal P-STE is recovered to the high level VH-STE at time T5, and makes the potential wells in the photo diodes 2 high. Then, the image-carrying light generates photo-carrier, and the photo-carrier is accummulated in the photo diodes 2, again.
The charge packets are transferred from the potential wells under the accumulation electrode 13 to the horizontal charge transfer element 11 during the time period T6, and are transferred to the output circuit 18 during time period T7.
The prior art area image sensor encounters a problem in distortion of the output potential signal from the output circuit 18. As shown in FIG. 6, while the horizontal charge transfer element 11 is transferring the charge packets to the output circuit 11, the anti-blooming signal P-AB sweeps the residual photo carrier into the anti-blooming drain regions 16, and the potential variation of the anti-blooming signal P-AB and the potential signals P-TGA/PSTE electrically affects the output potential signal from the output circuit 18. The output potential signal is deformed, and does not represent the image fallen onto the photo diode array 2.
It is therefore an important object of the present invention to provide a solid state image pickup device which is free from the influence of the anti-blooming operation.
It is also an important object of the present invention to provide a method of controlling the solid state image pick-up device.
The present inventor contemplated the problem, and noticed that a vertical overflow drain solved the problem. The vertical overflow drain directly swept excess photo carrier from photo diodes into the semiconductor substrate. However, when the vertical overflow drain was combined with the vertical charge transfer elements with the resistive gate electrode, the combination required high level driving signals. In detail, a solid state image pickup device with the vertical overflow drain required a potential signal read out from the photo-diodes to the vertical shift register higher than the row selecting signal. If large capacitance was required for the photo diodes, the read-out potential signal became much higher. The higher read-out potential signal resulted in the potential well under the accumulation electrode and the charge transfer region of the horizontal charge transfer element higher in potential level than those of the prior art, and, accordingly, the output circuit required a higher potential level in the reset drain. In order to maintain the potential level in the reset drain, it was necessary to make the amplitude of the potential signal on the accumulation electrode and the amplitude of the charge transfer signal for the horizontal charge transfer element wider than those of the prior art charge transfer device.
In this situation, the present inventor concentrated his efforts on a method of controlling a solid state image pickup device with a vertical overflow drain and resistive gate charge transfer units.
In accordance with one aspect of the present invention, there is provided a solid sate image pick-up device fabricated on a semiconductor device, comprising a plurality of photo-electric converting means for producing charge packets from an image-carrying light, a plurality of resistive gate charge transfer units having respective charge transfer channel regions and respective resistive gate electrodes capacitively coupled to the charge transfer channel regions, respectively, a plurality of first transfer gate elements having respective first transfer gate channel regions connected between the plurality of photo-electric converting means and the charge transfer channel regions and selectively changed between on-state and off-state for transferring certain charge packets to the charge transfer channel regions, respectively, a plurality of charge accumulating potential wells connectable to the charge transfer channel regions for accumulating the charge packets, a horizontal charge transfer unit electrically connectable to the plurality of charge accumulating potential wells for transferring the charge packets to an output circuit, a potential gradient producing means connected to first ends of the resistive gate electrodes and second ends of the resistive gate electrodes closer to the horizontal charge transfer unit than the first ends, and a vertical overflow drain formed under the plurality of photo-electric converting means for receiving excess charge from the plurality of photo-electric converting means.
In accordance with another aspect of the present invention, there is provided a method of controlling a solid state image pickup device including a plurality of photo-electric converting means for producing charge packets from an image-carrying light, a plurality of resistive gate charge transfer units having respective charge transfer channel regions and respective resistive gate electrodes capacitively coupled to the charge transfer channel regions, respectively, a plurality of first transfer gate elements having respective first transfer gate channel regions connected between the plurality of photo-electric converting means and the charge transfer channel regions and selectively changed between on-state and off-state for transferring certain charge packets to the charge transfer channel regions, respectively, a plurality of charge accumulating potential wells connectable to the charge transfer channel regions for accumulating the charge packets, a horizontal charge transfer unit electrically connectable to the plurality of charge accumulating potential wells for transferring the charge packets to an output circuit and a potential gradient producing means connected to first ends of the resistive gate electrodes and second ends of the resistive gate electrodes closer to the horizontal charge transfer unit than the first ends, and the method comprises the steps of a) making a potential level in the charge transfer channel region regions under the first ends higher than a potential level in the first transfer gate channel regions in the on-state so as to transfer the certain charge packets through the first transfer gate channel regions to the charge transfer channel regions, respectively, b) changing the switching elements to the off-state, and c) changing the potential level in the charge transfer channel regions under the first ends to a certain level lower than the potential level in the charge transfer channel regions under the first ends in the step a) and a potential level in the charge transfer channel regions under the second ends and higher than the potential level in the first transfer gate channel regions in the off-state so as to transfer the certain charge packets toward the plurality of charge accumulating potential wells.
In accordance with yet another aspect of the present invention, there is provided a method of controlling a solid state image pickup device including a plurality of photo-electric converting elements for producing charge packets from incident light, a plurality of resistive gate vertical charge transfer units having respective charge transfer channel regions and respective resistive gate electrodes capacitively coupled to the charge transfer channel regions, respectively, a plurality of first transfer gate elements having respective first channel regions connected between the plurality of photo-electric converting elements and the charge transfer channel regions and first transfer gate electrodes capacitively coupled to the first channel regions, respectively, for transferring certain charge packets from selected photo-electric converting elements to the charge transfer channel regions, respectively, a horizontal charge transfer unit electrically connectable to the charge transfer channel regions for transferring the charge packets to an output circuit and a controlling means connected to first ends of the resistive gate electrodes and second ends of the resistive gate electrodes closer to the horizontal charge transfer unit than the first ends, and the method comprises the steps of a) supplying a first potential level and a second potential level from the controlling means to the first ends and the second ends, respectively, so as to increase a potential difference between the selected photo-electric converting elements and the charge transfer channel regions, b) supplying a third potential level from the controlling means to the first transfer gate electrodes so that the charge packets are transferred from the selected photo-electric converting means through the first channel regions to the charge transfer channel regions, and c) supplying fourth potential from the controlling means to the first ends so as to increase a potential gradient along the charge transfer channel regions, thereby causing the plurality of resistive gate vertical charge transfer units to transfer the charge packets toward the horizontal charge transfer unit.